The archetypal image of the ' great cat who dwells at Heliopolis' probably did not refer to the domestic cat, but to th short-tailed jungle cat which lived in the thickets of the delta, for there is no reference to the domestic cat until the Eleventh Dynasty. Because it was hostile to snakes it became a sacred animal of the sun god. It is related in the Book of the Dead (chap.17) that the 'great cat' cuts off the head of the Apophis serpent whose body threatens the sacred per sea tree. In the New Kingdom the male cat was regarded as an incarnation of the sun-god and the she-cat was equated with the solar eye. Feline figures may display a scarab, the symbol of the rising sun, engraved on the head or breast thus showing their solar significance. The domestic cat attained special significance as the sacred animal of Bastet. Hundreds of figures of cats were set up as votive offerings in the temple at Bubastis in order that the donor might share in the godess's grace. Actual mummies of cats were buried in their thousands in special cemeteries in the area.

the reason is quite simple: cats are pest control. the egyptians held high respect for cats because they would prowl their granaries and storehouses, killing rats, mice, rabbits, and sometimes even snakes. thus, they protected both the egyptians and their food stores. overtime, this came to them being honored and eventually deified with the goddess Bastet (aka Bast or Baast, iirc)